This invention relates to a picture tube and more particularly to a color picture tube with an electron gun having non-circular apertures.
In a shadow mask type color tube, it is possible to converge three electron beams on a screen by means of a non-uniform magnetic deflection field without the use of a separate convergence coil. This deflecting field should consist of both a pincushion-shaped horizontal deflecting field for causing the three beams to be substantially converged along the horizontal axis and a barrel-shaped vertical deflecting field for causing the beams to be substantially converged along the vertical axis. While both the pincushion-shaped and barrel-shaped fields cause each of the three beams to focus in front of the phosphor screen on the side facing the electron gun (i.e., over focus) in the vertical direction, both fields cause each of the three beams to focus behind the phosphor screen (i.e., under focus) in the horizontal direction.
These focusing differences in both the horizontal and vertical directions affect the electron current distribution of the beam itself which causes coma aberration. The coma aberration due to the pincushion-shaped field can be overcome by adjusting a focusing voltage applied to the focusing electrode of the electron gun. The coma aberration however, caused by the barrel-shaped field remains unaffected.
As illustrated in FIG. 1, an electron beam 1 generated from an electron gun 2 has an electron current distribution comprising a high current density portion 3 at the center and a low current density portion 4 around the high current density portion 3. As the beam approaches the phosphor screen 5, it passes through the barrel-shaped vertical deflection field 6. In the vertical direction, the electrons positioned near the center of the deflecting field 9 are subject to a stronger deflecting force than the electrons positioned away from the center of the field. Therefore, the beam will be focused in front of the screen as shown by focal point 7 (i.e., over focus). Consequently, the beam spot on the screen at the periphery thereof is elongated, and comprises a high current density portion 3 and a tail-shaped haze portion 8 around the high density portion 3.
Reducing the coma aberration can be obtained by using a large lens or a complex focusing lens. Any reduction in aberration, however, produced by such structure will be restricted by the size of the electrodes that can be used with the narrow neck color picture tubes currently used.
The instant invention overcomes coma aberration by utilizing an electron gun which comprises a triode section and a focusing section; each of these sections being provided with at least one electrode containing a non-circular aperture having major and minor axes of symmetry. An electrostatic focusing lens formed by electrodes contains a non-circular aperture electrode within the focusing section for elongating the focal length in the vertical direction (i.e., vertical focal length) to a greater degree than the focal length in the horizontal direction (i.e., horizontal focal length).
The major axis of the non-circular aperture within the triode section is positioned substantially parallel to the horizontal direction. While the use of non-circular apertures are known in the prior art, they have been used other than disclosed by the instant invention. For example, U.S. Pat. No. 3,497,763 discloses the use of a single non-circular aperture in a picture tube to compensate for the astigmatic effect caused by a quadrupolar lens used for deflection amplification. U.S. Pat. No. 3,524,094 discloses a picture tube which generates a narrow elongated spot and the use of a single non-circular aperture to minimize the twist of the spot during deflection. British Pat. No. 1,421,865 discloses the use of two noncircular aperture in the triode of the electron gun to obtain a narrow elongated spot for an indexing-type tube.